Butterfly valve

ABSTRACT

An improved butterfly valve of the type wherein a disc mounted for rotation about a nondiametric axis and having a continuously convergent circumferential sealing surface which cooperates with a wall-mounted annular flexible seat is disclosed. The inwardly projecting nature of the flexible seat subjects it to highly erosive forces when fluid is throttled between it and the disc when the disc is opened to a throttling position, i.e. approximately 15* from the closed position or greater. To protect the seat from such erosive forces, the rigid wall of the fluid flow channel is projected inwardly on both sides of the seat to provide a far more erosion-resistant surface for throttling operation. These same inward projections can also be employed to provide a scraping lip which will remove solids buildup from the sealing surface of the disc as it is rotated toward the seat. A manner of affixing the disc to the shaft without providing a leak path across the disc and whereby the forces imposed by fluid pressure upon the disc are transmitted directly to the shaft is also disclosed. This entails passing the shaft through a shaft bore in an integrally formed gudgeon in said disc and passing tapered pins parallel to the surfaces of the disc through the gudgeon and shaft in such a manner that a majority of, but not the entire circumference of, the tapered pin lies within the circumference of the shaft. Rotation of the disc beyond the desired one-quarter turn from the fully closed to the fully open position and displacement of the disc along the direction of the axis of the shaft are prevented by lugs extending into the fluid flow channel from the valve body.

United States Patent IHI 3,591,133

[721 Inventors MichaelE.Miles Northborough; James F. Donnelly.Worcester. both of. Mass. [21] ApplNo 754,925 [22] Filed Aug.23, 1968[45] Patented July/6,1971 [73] Assignee Jamesbury Corporation Worcester,Mass.

[54] BUTTERFLY VALVE 2 Claims, 4 Drawing Figs.

[52] U.S.C1 251/173, 251/210,2S1/306,251/308 [51] lnt.C1 ..Fl6k13/02[50] FicldofSearch 251/306, 286, 173, 306, 308; 137/242; 187/5208 [56}References Cited UNITED STATES PATENTS 688,860 12/1901 Kayetal207/52.08X 1,905,684 4/1933 Coffman..... 25l/286X 2,586,927 2/1952 Fantz251/306 2,893,682 7/1959 l-lintzman 251/173 2,980,388 4/1961 White25l/306UX 3,288,163 11/1966 -Craven.... 137/242 3,304,050 2/1967 Fawkes25l/173X I TD)IDYDIYIIIIIIA a Primary Examiner Harold W WeakleyAtt0rneys-Robert F Conrad and Kurt Shaffert ABSTRACT: An improvedbutterfly valve of the type wherein a disc mounted for rotation about anondiametric axis and having a continuously convergent circumferentialsealing surface which cooperates with a wall-mounted annular flexibleseat is disclosed. The inwardly projecting nature of the flexible seatsubjects it to highly erosive forces when fluid is throttled between itand the disc when the disc is opened to a throttling position, i.e.approximately 15 from the closed position or greater. To protect theseat from'such erosive forces, the rigid wall of the fluid flow channelis projected inwardly on both sides of the seat to provide a far moreerosion-resistant surface for throttling operation. These same inwardprojections can also be employed to provide a scraping lip which willremove solids buildup from the sealing surface of the disc as it isrotated toward the seat. A manner of affixing the disc to the shaftwithout providing a leak path across the disc and whereby the forcesimposed by fluid pressure upon the disc are transmitted directly to theshaft is also disclosed. This entails passing the shaft through a shaftbore in an integrally formed gudgeon in said disc and passing taperedpins parallel to the surfaces of the disc through the gudgeon and shaftin such a manner that a majority of, but not the entire circumferenceof, the tapered pin lies within the circumference of the shaft. Rotationof the disc beyond the desired one-quarter turn from the fully closed tothe fully open position and displacement of the disc along the directionof the axis of the shaft are prevented by lugs extending into the fluidflow channel from the valve body.

PATENTED JUL 6197! SHEET 1 OF 2 JAMES fiOA A/ELL y PATENTEUJUL BIB?!3,591,133 I SHEEIBUFZ Has - /-vs-rae s Mama fM/zzs JAMES EfldzV/VELLYBUTTERFLY VALVE BACKGROUND OF THE INVENTION This invention relates toimproved rotary valves for the control of fluid flow and morespecifically to butterfly valves. In this type of valve a valve disc orbutterfly is mounted in the valve body to rotate about an axis from anopen position in which the disc lies parallel to the flow axis to aclosed position in which it lies perpendicularly to the flow axis. Moreparticularly, the invention relates to the type of butterfly valve inwhich a flexible seat is disposed around the periphery of the flowchannel for coaction with a sealing surface on the circumference of thebutterfly disc when the latter is rotated into the closed position.

The invention is particularly applicable to valves of this type in whichinternal fluid pressure serves to improve the coaction of the seat withthe sealing surface on the butterfly.

Butterfly valves of the type in which a flexible seat projects into thefluid flow channel from the periphery thereof are disclosed in theHintzman et al. U.S. Pat. No. 2,893,682 and the White U.S. Pat. No.2,980,388. Although valves such as this and others known to the priorart are useful in certain types of service, problems of seat life arisein strenuous service from the fact that the flexible seat is exposed tothe high-pressures and rapid flow rates that prevail in the fluidstream. During the act of closing the valve, as the butterfly approachesthe seat, the erosive effect of fluid rushing with ever-increasingvelocity past the flexible seat as the flow area is throttled downaccentuates these harmful effects. Also, where the nature of the fluidpassing through the valve is such that solid particles may collect onthe sealing surface of the butterfly disc, these particles will tend todamage the flexible seat when the valve is closed.

Butterfly valves are also employed for throttling service, i.e. servicewherein the valve is used to reduce the fluid pressure to somepredetermined fraction (not greater than 1, obviously) of thatprevailing upstream. lt has been found that butterfly valves arereliable for throttling service at openings such as are provided byrotation of the disc of at least approximately from the closed position.At disc positions opened to a substantially lesser extent the valvetends to be unstable due to fluid vortexing. At disc openingssubstantially greater than 70 degrees from the closed position,virtually full flow is attained. Thus the effective range for throttlingis disc rotations on the order of 15 to 70 from the closed position,plus or minus about 5. Thus it is particularly desirable to protect theseat from the erosive forces produced by fluid throttling in this rangeof disc openings.

Another problem frequently encountered with butterfly valves of the typedescribed known to the prior art stems from the fact that fluid pressureacting on the bottom of the shaft is essentially unopposed by fluidpressure acting upon any other surface in the opposite direction andthus creates a resultant upward force upon the shaft. With the butterflyrigidly attached to such shaft any upward movement of the shaft createdby such unbalanced force results in the removal of the butterfly fromits position of optimum cooperation with the flexible seat.

It is an object of this invention to provide a butterfly valve of thetype described wherein no displacement of the butterfly from itsposition of optimum cooperation with the flexible seal means occurs dueto the action of the said resultant upward force upon the shaft.

Where a valve is employed to control the flow of flammable fluids it ishighly desirable that its structure permits it to provide at least arelatively effective seal even if the valve has been exposed to quitehigh temperatures, such as would prevail during a fire. Otherwise thefire, which tends to char and destroy the flexible seat and otherorganic elements of a valve, may release the flow of the flammable fluidand thereby increase the ferocity of the fire.

It is still another object of this invention to provide a butterflyvalve of the type described wherein a relatively effective seal can bemaintained despite'heat damage to the seat an other organic elementsthereof.

Other advantages and objectives of this invention and the manner oftheir attainment will be apparentto those skilled in this art from aconsideration of this specification when taken in conjunction with theaccompanying drawing.

SUMMARY In butterfly valves of the type described, the necessity forprojecting the radially inwardly portion of the annular seat into theflow channel creates a situation where the seat is readily subjected toexcessive wear conditions. Inasmuch as it is required, in valves of thistype, that surfaces of the seat which are noncolinear with the sealingsurface thereof be exposed to the fluid stream so that the fluidpressure may enhance the sealing effectiveness of the seat, it is notpossible to protect the seat by disposing all but the sealing surfacethereof within the valve body and out of the flow channel as is done forinstance in the valve disclosed by Bryant in U.S. Pat. No. 3,000,609.

According to the instant invention, an annular rigidsurfaced ringelement may be disposed upstream of the seat. It is desirable that thiselement extend-radially inward as far, or at least nearly as far, intothe fluid stream as the radially inwardmost segment of the seat itself.Thus, the annulus of the said ring element should not be substantiallylarger than that of the seat. The thus placed ring element willmaterially reduce the deleterious effect of direct impingement by therapidly flowing fluid upon the portion of the seat projecting into theflow channel. The ring element may also serve to protect the seat fromdirect. high-velocity impingement by particles which may be entrained inthe fluid stream. Where a butterfly valve is to be used in service inwhich fluid flow may alternatively occur in either direction, a ringelement of the type described is employed on either side of the seat.This provides dual advantages. Firstly, the seat is provided with thetype of protection described above from whichever ring element happensto be in the upstream location. Secondly, the instant invention may beused to optimum advantage in protecting the seat during throttling ofthe fluid stream where ring elements upstream and downstream of the seatare employed. When the valve is employed to throttle the fluid stream,fluid passes through a flow channel of considerably smallercross-sectional area than when the valve is set for full flow. Theattendant increased linear fluid velocities impose highly erosive forcesupon the surfaces which bound this reduced cross-sectional area. Thus,where throttling occurs between the sealing surface of the disc on theone hand and the seat on the other hand, these erosive forces act uponthe flexible seat. Where rigid surfaces are projected into the fluidflow channel on either side of the seat so that these rigid surfaces arecloser to the sealing surface of the disc when the valve is in athrottling position (i.e. approximately 15 or greater from the closedposition) these surfaces will form the boundary of the reducedcross-sectional area flow channel, thereby diverting the erosivethrottling forces from acting upon the seat. It will be seen from aconsideration of the geometry of these valves that at any throttlingposition of the disc two points on the circumference of the seat mustnecessarily be closer to the sealing surface of the disc than any pointon the respective ring elements, so that this beneficial effect of theinstant invention can not be made to apply to all portions of thecircumference of the seat simultaneously. However, the two thusunprotected points on the circumference of the seat will be in adifferent location on the seat for every different throttling setting ofthe disc, so that even this deleterious effect is ameliorated bydistribution about the circumference of the seat during typical use of avalve. It is of course necessary that the ring elements do not projectinto the flow channel so far as to intersect the locus of rotation ofthe disc. However, it is desirable that the annulus of the ring elementscome as close as possible to this locus. If thediameter of the annulusof the ring element is sufficiently small to assure that every point onthat portion of the circumferential sealing surface of the disc which isrotated toward that particular ring element (as opposed to that portionof the circumferential sealing surface which is rotated toward the otherring element) in opening the valve is closer to that ring element thanto any portion of the flexible seat, the advantages of the invention inprotecting the seat during fluid throttling will be attained. It willalso apparent to one skilled in this art from a consideration of thisspecification and drawing that these advantages can also be realizedwithout providing fully circumferential annular ring elements, but byemploying suitable disposed rigid-walled projections into the fluidchannel.

Protection of the radially inwardly projecting flexible seat required invalves of the instant type against other forms of damage are alsoprovided by the instant invention. Thus, closing of the valve when thedisc has been displaced in the direction along the axis of the shaftwould tend seriously to mar or gall the seat. Likewise, rotating thedisc fully about its axis of rotation can cause such harm. This latterdanger exists especially in valves of this type wherein the disc ismounted for rotation about an axis other than a diameter of the disc.Such nondiametric mounting of the disc for rotation is especiallydesirable where the circumferential seating surface is a continuouslyconverging surface. Conveniently, this surface may be a segment of asphere having its center on the axis of the flow channel through thevalve, or a frustum of a cone having its apex on the same axis. Theadvantage of mounting discs having such circumferential sealing surfacesnondiametrically is that when rotated, the circumferential sealingsurface on such a disc moves away from the seat at all points along thecircumference thereof, rather than scraping across the seat during eachopening and closing.

It can readily be appreciated that the full rotation of such anondiametrically mounted disc could seriously damage the seat. Accordingto this invention, the valve body is provided a similar heat destructiontemperature as that of the seat and having a thickness of the same orderof magnitude as the clearance between the locus of rotation of the discand the annulus of the ring element, substantially effective metal tometal sealing can be provided between the disc and the.ring element inthe event' of the heat failure of the seat and the bearings, theupstream fluid pressuredeflecting the disc and shaft downstream intothis metal to metal seatingposition.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevation of a preferredembodiment of thein- 'vention;

DESCRIPTION OF A PREFERRED EMBODIMENT OF v THE INSTANT INVENTION As maybest be seen in FIGS. 1 and 2, valve body 10 defines by its internalwalls 11 a fluid flow channel 12 having an axis 13. The valve housingmay be formedof any suitable material of construction, as for instancean appropriate metal such as stainless steel. i

The valve body 10 also has cavities 14 and 15 disposed therein,colinearly aligned at opposite ends of flow channel 12. As may best beseen in FIG. 1, the axis 16 of these cavities is displaced slightly fromthe diameter 17 of the flow channel. Therefore this axis, and the axisof the shaft to be mounted in I these cavities, extends in the manner ofa secant other than a with lugs projecting into the fluid flow channelwhich prevent Q the rotation of the disc through a substantially greaterextent than the 90 rotation required to go from'a fully closed to afully open position.

These same lugs may be positioned in accordance with this invention toprevent the displacement of the disc in either direction along the axisof the shaft on which the disc is mounted.

Rigid attachment of the disc to the shaft for rotation therewith is alsoan important feature of a design whereby precise positioning of the discwith respect to the seat assures that the valve will not be subject toseat damage due to improper cooperation between the circumferentialsealing surface of the disc and the seat.

, According to the instant invention the disc has a shaft bore passingthrough an element, such as a gudgeon, formed integrally therewith. Theshaft is secured within the shaft bore by means which require no opening(and thus provide no potential leak path) between one face of the discand the other. Suitable fastening may be provided by at least onetapered pi'n passing through the gudgeon and shaft bore parallel to theface of the disc in such a manner that a majority of, but not all of,the circumference of the tapered pin passes through the shaft. In thismanner, the pin compresses the shaft against the opposite side of theshaft bore rather than being placed'in shear at both surfaces where itenters and leaves the shaft, as would be the case if the pin passedcentrally through the shaft.

A special advantage of the instant invention is that it may befireproof" service. Ordinarily, butterfly valves fail when subjected totemperatures at which the organic components thereof, such as the seatand possibly portions of the bearings lose their structural properties.This temperature may be referred to as the heat destruction temperature.By mounting the shaft of a valve of the instant invention in bearingshaving .employed in the design of butterfly valves for so-called ndiameter, or nondiametrically, across thefluid flow channel.

Shaft 24) extends transversely across fluid flow channel 12,

perpendicular to the axis 13 thereof. Upper and lower shaft bearings 21and 22, respectively, are disposed in cavities 14 and 15, respectively.These bearings are sleeve bearings and may conveniently made ofself-lubricating plasticmetal laminates such as TeflohReginteredTrademlrk, E. I. duPdnt de Nemours and Company forpnlyuetrafluoroethylcnc). impregnated perforated stainless steel. Thethickness dimenlion of these hearings may be carefully controlled toprovide the fireproof valve in this r Disc 30 has parallel, circularfaces 31 and 32 and is bounded by a continuously convergentcircumferential sealing surface 33. In the embodiment shown, surface 33is a frustum of a cone having its apex on axis 13 (extended), but otherconvergent surfaces, such as segments of spheres, may also be used.Formed integrally with disc 30 is gudgeon 34, having shaft bore 35adapted to receive shaft 20 passing therethrough. Tapered pinholes 36and 37 pass through said gudgeon parallel to faces 31 and 32 of saiddisc. As may best be seen in FIG. 3, said pinholes 36 and 37 passthrough gudgeon 34 in such a fashion as to intersect nontangentiallywith shaft bore 35 so that when tapered pins 36p and 37p are placedtherein a majority, but not all, of the circumference of said taperedpins will pass through shaft 20, assuring that shaft 20 will becompressed firmly against shaft bore 35. By this arrangement theresultant force of the fluid pressure acting on disc 30 is transmittedfrom the disc directly to shaft 20 (and thence to shaft bearings 14 and15) without the buildup of stresses at the shear planes where thetapered pins enter and leave the shaft, as would be the case werethetapered pins to pass centrally through the shaft.

Lugs 38 and 38a projecting into flow channel 12 serve to preventrotation of disc 30 beyond the fully open position. Lugs 39 and 39aserve to prevent rotation of disc 30 in the wrong direction from thefully closed position. Lugs 38 and 39 also serve to prevent the upwarddisplacement of disc 30 along the direction of the axis of shaft 20 andlugs 38a and 39a similarly prevent the opposite displacement of disc 30.

Seat 40 is fashioned of stiffly flexible material such as Teflon orother polyfluorocarbon, nylon, polyethylene,

rubber, rubber-resin blends or other suitable compositions. v

The radially outward portion 41 thereof is embedded in valve body 10.For ease of replacement, this embedding may be accomplished by the useof a removable retainer ring 42 which is conveniently made of the samemetal as valve body 10.

The radially inward portion 43 of seat 40 has an annular sealing surface44 as well as fluid contacting surfaces 45 and 46. As may better be seenin FIG. 4, these fluid contacting surfaces are noncolinear with sealingsurface 44. Fluid pressure acting on these fluid-contacting surfaces mayserve to deflect the radially inward portion 43 of seat 40 and therebyenhance the effectiveness of the sealing action between seat 40 andcircumferential sealing surface 33 of disc 30.

Rigid-surfaced ring elements 50 and 51 are shown displaced in eitherdirection along the axis of flow channel 12 from seat 40. As may best beseen in FIG. 4, these surfaces project radially inwardly close to thelocus of rotation of disc 30 without intersecting it. Sharply angularlips 50(1) and 51(1) are formed at that circumferential element of eachof these surfaces which is closest to that locus of rotation.

As may also be seen in FIG. 4, the rotation of disc 30 (as indicated byphantom lines) from the fully closed position toward throttlingpositions creates a condition where disc 30 is closer to surfaces 50 and51 than to any point on seat 40, so that fluid throttling occurs betweenthe rigid surfaces of disc 30 and rigid surfaces 50 and 51 rather thanbetween disc 30 and flexible seat 40.

On the basis of this disclosure of the invention, that which we claimis:

l. A butterfly valve comprising a valve body of rigid material defininga circular cross-sectional fluid flow channel therethrough,

an annularseat affixed by the radially outwardly portion thereof in saidvalve body to circumscribe said flow channel, said seat having aflexible radially inwardly portion protruding into said flow channel,

a shaft aligned nondiametrically across said flow channel at a pointaxially displaced along said flow channel from said annular flexibleseat,

a valve disc mounted for rotation about the axis of said shaft,

the circumferential surface of said disc constituting a segment of acontinuous geometric surface converging at a point on the axis of saidflow channel, said surface constituting a sealing surface adapted tocooperate with said annular seat,

the radially inwardly portion of said annular flexible seat having anannular sealing surface adapted to cooperate with said circumferentialsealing surface on said disc to block flow through said channel whensaid disc is rotated into the closed position perpendicular to the axisof said flow channel,

another annular surface of said inwardly portion of said annularflexible seat, disposed noncolinearly to said annular sealing surfacethereof, exposed to the fluid in said flow channel and adapted toaugment the sealing effectiveness of said seat against said disc by theaction of fluid pres sure,

said valve effectively controlling the flow of fluid therethrough at anydesired proportion not greater than unity of the fluid pressureprevailing upstream thereof by rotation of said disc about thenondiametric axis to any flow-controlling position at least about fromsaid closed position,

an annular rigid-surfaced ring element axially displaced along said flowchannel from said shaft and from said annular seat, but proximate to thelatter, the annular opening thereof projecting radially inward to anextent short of intersecting with the locus of rotation of said disc butsufficient to'assure that, when said disc is in said flow-controllingposition, every point on said circumferential sealing surface on thatportion of the disc rotated in, the axial direction toward said ringelement is closer to said ring element than to any portion of saidflexible seat,

and another said ring element being positioned on the opposite side ofsaid seat from the first said ring element,

the respective extremities of said shaft being mounted in narrow-walledsleeve bearings inserted into cavities in said valve body, the materialsfrom which said sleeve bearings and said flexible portion of saidannular seat are formed having a heat-destruction temperatureconsiderably lower than that of said valve body and said disc,

the wall thickness of said sleeve-bearings being substantially the sameas the clearance between the locus of rotation of said disc and saidring elements,

whereby substantial sealing effectiveness is retained by said valvedespite heat damage thereto.

2. A butterfly valve comprising a valve body of rigid material defininga circular cross-sectional fluid flow channel therethrough,

an annular seat affixed by the radially outwardly portion thereof insaid valve body to circumscribe said flow channel, said seat having aflexible radially inwardly portion protruding into said flow channel,

a shaft aligned nondiametrically across said flow channel at a pointaxially displaced along said flow channel from said annular flexibleseat,

a valve disc mounted for rotation about the axis of said shaft,

the circumferential surface of said disc constituting a segment of acontinuous geometric surface converging at a point on the axis of saidflow channel, said surface constituting a sealing surface adapted tocooperate with said annular seat,

the radially inwardly portion of said annular flexible seat having anannular sealing surface adapted to cooperate with said circumferentialsealing surface on said disc to block flow through said channel whensaid disc is rotated into the closed position perpendicular to the axisof said flow channel,

another annular surface of said inwardly portion of said annularflexible seat, disposed noncolinearly to said annular sealing surfacethereof, exposed to the fluid in said flow channel and adapted toaugment the sealing effectiveness of said seat against said disc by theaction of fluid pressure,

said valve effectively controlling the flow of fluid therethrough at anydesired proportion not greater than unity of the fluid pressureprevailing upstream thereof by rotation of said disc about thenondiametric axis to any flow-controlling position at least about 15from said closed position,

an annular rigid-surfaced ring element axially displaced along said flowchannel from said shaft and from said annular seat, but proximate to thelatter, the annular opening thereof projecting radially inward to anextent short of intersecting with the locus of rotation of said disc butsufficient to assure that, when said disc is in said flow-controllingposition, every point on said circumferential sealing surface on thatportion of the disc rotated in the axial direction toward said ringelement is closer to said ring element than to any portion of saidflexible seat,

a plurality of lugs, projecting into said fluid channel from said valvebody, to prevent the rotation of said disc substantially beyond aquarter-turn rotation from the fully closed position to the fully openposition, whereby avoidance of damage to the flexible annular seatcaused by rotation of the nondiametrically mounted disc in the wrongdirection is assured,

ible seat caused by seating of the disc when improperly positioned alongthe said direction

1. A butterfly valve comprising a valve body of rigid material defininga circular cross-sectional fluid flow channel therethrough, an annularseat affixed by the radially outwardly portion thereof in said valvebody to circumscribe said flow channel, said seat having a flexibleradially inwardly portion protruding into said flow channel, a shaftaligned nondiametrically across said flow channel at a point axiallydisplaced along said flow channel from said annular flexible seat, avalve disc mounted for rotation about the axis of said shaft, thecircumferential surface of said disc constituting a segment of acontinuous geometric surface converging at a point on the axis of saidflow channel, said surface constituting a sealing surface adapted tocooperate with said annular seat, the radially inwardly portion of saidannular flexible seat having an annular sealing surface adapted tocooperate with said circumferential sealing surface on said disc toblock flow through said channel when said disc is rotated into theclosed position perpendicular to the axis of said flow channel, anotherannular surface of said inwardly portion of said annular flexible seat,disposed noncolinearly to said annular sealing surface thereof, exposedto the fluid in said flow channel and adapted to augment the sealingeffectiveness of said seat against said disc by the action of fluidpressure, said valve effectively controlling the flow of fluidtherethrough at any desired proportion not greater than unity of thefluid pressure prevailing upstream thereof by rotation of said discabout the nondiametric axis to any flowcontrolling position at leastabout 15* from said closed position, an annular rigid-surfaced ringelement axially displaced along said flow channel from said shaft andfrom said annular seat, but proximate to the latter, the annular openingthereof projecting radially inward to an extent short of intersectingwith the locus of rotation of said disc but sufficient to assure that,when said disc is in said flow-controlling position, every point on saidcircumferential sealing surface on that portion of the disc rotated in,the axial direction toward said ring element is closer to said ringelement than to any portion of said flexible seat, and another said ringelement being positioned on the opposite side of said seat from thefirst said ring element, the respective extremities of said shaft beingmounted in narrow-walled sleeve bearings inserted into cavities in saidvalve body, the materials from which said sleeve bearings and saidflexible portion of said annular seat are formed having aheat-destruction temperature considerably lower than that of said valvebody and said disc, the wall tHickness of said sleeve bearings beingsubstantially the same as the clearance between the locus of rotation ofsaid disc and said ring elements, whereby substantial sealingeffectiveness is retained by said valve despite heat damage thereto. 2.A butterfly valve comprising a valve body of rigid material defining acircular cross-sectional fluid flow channel therethrough, an annularseat affixed by the radially outwardly portion thereof in said valvebody to circumscribe said flow channel, said seat having a flexibleradially inwardly portion protruding into said flow channel, a shaftaligned nondiametrically across said flow channel at a point axiallydisplaced along said flow channel from said annular flexible seat, avalve disc mounted for rotation about the axis of said shaft, thecircumferential surface of said disc constituting a segment of acontinuous geometric surface converging at a point on the axis of saidflow channel, said surface constituting a sealing surface adapted tocooperate with said annular seat, the radially inwardly portion of saidannular flexible seat having an annular sealing surface adapted tocooperate with said circumferential sealing surface on said disc toblock flow through said channel when said disc is rotated into theclosed position perpendicular to the axis of said flow channel, anotherannular surface of said inwardly portion of said annular flexible seat,disposed noncolinearly to said annular sealing surface thereof, exposedto the fluid in said flow channel and adapted to augment the sealingeffectiveness of said seat against said disc by the action of fluidpressure, said valve effectively controlling the flow of fluidtherethrough at any desired proportion not greater than unity of thefluid pressure prevailing upstream thereof by rotation of said discabout the nondiametric axis to any flow-controlling position at leastabout 15* from said closed position, an annular rigid-surfaced ringelement axially displaced along said flow channel from said shaft andfrom said annular seat, but proximate to the latter, the annular openingthereof projecting radially inward to an extent short of intersectingwith the locus of rotation of said disc but sufficient to assure that,when said disc is in said flow-controlling position, every point on saidcircumferential sealing surface on that portion of the disc rotated inthe axial direction toward said ring element is closer to said ringelement than to any portion of said flexible seat, a plurality of lugs,projecting into said fluid channel from said valve body, to prevent therotation of said disc substantially beyond a quarter-turn rotation fromthe fully closed position to the fully open position, whereby avoidanceof damage to the flexible annular seat caused by rotation of thenondiametrically mounted disc in the wrong direction is assured, saidlugs being positioned to interfere with the displacement of said discalong the direction of the axis of said shaft, thereby assuringavoidance of damage to the annular flexible seat caused by seating ofthe disc when improperly positioned along the said direction.